1. Field of the Invention
This invention relates to epoxidized polyamide wet strength resins containing lecithin and their use in paper and molded pulp products made of cellulose fibers such as wood pulp.
2. Background and the Prior Art
In the manufacture of wet strength paper and molded pulp products, a wet strength resin is added to the pulp slurry. Wet strength resins are typically of the epoxidized polyamide, urea formaldehyde or melamine formaldehyde types. These resins provide cross-linking to impart wet strength required by various paper and molded pulp products.
During the paper or molded pulp drying process, polymers such as those of melamine formaldehyde and urea formaldehyde may become a health hazard due to release of formaldehyde. Also, the epoxidized polyamide polymers as well as the melamine formaldehyde and urea formaldehyde polymers can at times stick to hot dryer surfaces. This problem is particularly acute with the epoxidized polyamide wet strength resins in the making of molded pulp products. Thus, in the manufacture of molded pulp products, wherein melamine formaldehyde wet strength resin is used, we have found that sticking is not a problem. However, the use of epoxidized polyamide in place of the melamine formaldehyde caused undesirable sticking of resin and pulp to the drier and furthermore the internal bond strength of the dried molded pulp product was weak. Also, it appears that urea formaldehyde wet strength resins are also not as susceptible to sticking to heated drier surfaces as with the epoxidized polyamides.
The application of various release agents to paper making dryer surfaces as well as to heated platens in pressing glue coated wood particles to make panels is well known for preventing sticking of resin to such surfaces. However, such application of a surface lubricant means the addition of another process step with the consequent increase in production time as well as an additional cost due to the amount of lubricant needed. Also, for release of molded pulp products from molds, additional difficulties are encountered in application of release agents due to the contoured and curvilinear surfaces of such molds.
In the making of wood based panels such as particleboard, by using melamine formaldehyde glues, press operators have applied an emulsion of five parts of lecithin in four parts aqua ammonia of 26 Baume and 91 parts of water as a release agent on the press surfaces. Such release agent is sold by Borden, Inc. under the designator PC-803L.
U.S. Pat. No. 4,076,896 of Feb. 28, 1978, shows the manufacture of laminates by impregnating paper with a melamine formaldehyde glue containing lecithin wherein the lecithin increases the release characteristics of the resin when pressing out a panel. Japanese patent publication JP55-139430 to Matsushita Elec. Works relates to the manufacture of a laminated sheet which includes impregnating paper or cloth with a thermosetting resin containing lecithin wherein the resins are said to include phenol resins, epoxy resins, polyester resins and melamine resins. U.S. Pat. No. 4,267,240 of May 12, 1981 to Formica Corp. relates to a release sheet comprising a web of paper having one side coated with various materials including lecithin.
The Kamikaseta et al U.S. Pat. No. 4,634,727 of Jan. 6, 1987 relates to a polyvinyl acetate emulsion adhesive for bonding wood and other porous substances wherein lecithin is added directly to the adhesive emulsion or the lecithin is first emulsified with aqueous ammonia before addition to the adhesive. In this 727 patent, the lecithin is said to assist in release of the adhesive from the press platens and increases the bonding strength of the adhesive. The 727 patent states that various additional polymers may be added to the lecithin containing polyvinylacetate such as urea resin, phenol formaldehyde resin and melamine resin.
Japanese patent publication, JP 1045894 to OJI Paper KK relates to the manufacture of paper which is said to have improved releasability by having release agents added to a layer of paper wherein the release agents include lecithin. Japanese Patent publication JP 88-057206 to Kobe relates to the production of a laminate by preimpregnating a paper substrate with a solution containing a surfactant and a cure accelerator for a phenolic resin varnish wherein lecithin is referred to as a cure accelerator. U.S. Pat. Nos. 1,977,251 to Stallmann of Oct. 16, 1934 and 3,947,383 to Baggett of Mar. 30, 1976 describe reaction products of ammonia and epichlorohydrin for use as a paper wet strength resin additives.